Add Z-index to layers

This commit allows setting a z-index property on the layers and uses
it in the canvas, WEBGL and DOM map renderers for ordering the layers
before rendering.

Default z-index is 0 for managed layers and 1000 for unmanaged ones.
It allows always on bottom, always on top and more complex layer layouts.

src/ol/layer/layer.js

@@ -172,6 +172,7 @@ ol.layer.Layer.prototype.setMap = function(map) {
         map, ol.render.EventType.PRECOMPOSE, function(evt) {
           var layerState = this.getLayerState();
           layerState.managed = false;
+          layerState.zIndex = Infinity;
           evt.frameState.layerStatesArray.push(layerState);
           evt.frameState.layerStates[goog.getUid(this)] = layerState;
         }, false, this);